# -*- coding: utf-8 -*-

def merge_sort(alist):
	"""归并排序"""
	n = len(alist)
	if n <= 1:
		return alist
	mid = n / 2
	# left_li 采用归并排序后形成的有序的新的列表
	left_li = merge_sort(alist[:mid])
	# right_li 采用归并排序后形成的有序的新的列表
	right_li = merge_sort(alist[mid:])
	# 将两个有序的子序列合并为一个整体，利用两个指针
	left_pointer, right_pointer = 0, 0
	result = []
	
	while left_pointer < len(left_li) and right_pointer < len(right_li):
		if left_li[left_pointer] <= right_li[right_pointer]:
			result.append(left_li[left_pointer])
			left_pointer += 1
		else:
			result.append(right_li[right_pointer])
			right_pointer += 1
			
	result += left_li[left_pointer:]
	result += right_li[right_pointer:]
	return result

if __name__ == "__main__":
	li = [54, 26, 93, 17, 77, 31, 44, 55, 20]
	print li
	sorted_li = merge_sort(li) 
	print sorted_li